Multiple radio receiving system



y 1934- c. B. AIKEN 1,957,519

MULTIPLE RADIO RECEIVING SYSTEM Filed Dec. 18, 1931 lNVENTOR C. 8.14/KENIBY%%M A TTORNEV Patented May 8, 1934 UNITED STATES PATENT OFFICECharles B. Aiken, New

Bell

York, N. Y., assignor to Telephone Laboratories, Incorporated,

New York, N. Y., a corporation of New York Application December 18,1931, Serial No. 581,876

2 Claims.

This invention relates to the multiple connection of radio receivingsystems and in particular to the parallel connection of a plurality ofradio receivers to a common antenna.

5 A multiple connection of this sort may require that a large number ofradio receivers be connected and operated simultaneously and that allthe receivers may at times be tuned to one i and the same station. It isdesirable that the tuning of any receiver or receivers shall notmaterially affect either the tuning or the volume of response from anyother receivers. It is also desirable that, when a receiver havingmultiple stage tuning with unified control is used, the

coupling of the other receivers to the system will not interfere withthe functioning of the tuning mechanism by detuning the input circuitand thereby disturbing the alignment of the several stages. The multipleconnection of the receivers should provide as great sensitivity as isconsistent with a tolerable amount of interaction between the separatereceivers and a minimum of interference with the proper action of thetuning mechanism.

In accordance with the present invention the input circuit of eachreceiving set of a connected group is modified by the addition of alarge series inductance through which it is connected to the V secondaryof a common transformer, the primary of which is connected to an antennaor high frequency transmission line. The transformer is designed toeffect a satisfactory impedance match between the antenna circuit andthe assemblage of radio receivers. The series inductance provides aloose coupling between the several receivers and between each individualreceiver and the antenna and also serves to equalize the sensitivity ofthe receiver over a wide range of frequencies.

49 The invention is described in detail hereinafter in connection withthe accompanying drawing in which the single figure shows a plurality ofradio receiving sets connected to a common antenna in accordance withthe invention.

Referring to the drawing, a plurality of radio receiving sets 1, 1 areconnected between a shielded high frequency transmission line 10 and theinput circuit 11 of an individual radio receiver 2. The transmissionline comprises a central conductor 12 associated with an antenna 13 anda concentric tubular shield 14 connected to ground at 15. The inputcircuit 11 includes a potentiometer 16, a series inductance 17 and atransformer 18. The secondary winding 19 of the transformer 18 is tunedby means of a variable condenser 20 to resonate with signals of anydesired frequency impressed on the antenna 13. The remainder of thereceiving set 2 is of the conventional type and need not be completelydisclosed as it is not a part of the present invention. There is shownonly the first vacuum tube 21, the final detector 22 and a receiver orloud speaker 23. The ouiput of the tube 21 includes a tuned circuitcontaining a variable condenser 24 which is mechanically coupled to thevariable condenser 20 to permit the simultaneous tuning of therespective resonant circuits.

The receivers l, l and 2 are connected in parallel with each other andare associated with the antenna through transformers 26 and 27 and thetransmission line 10. The transformer 26 is designed to provide asatisfactory impedance match between the antenna and the transmissionline. The impedance raio of the transformer 27 is to be determined inview of the actual number of receivers l, 1 and 2 which will be used ina given installation. The transformer should provide a substantialimpedance match between the transmission line 10 and the combinedimpedance of the parallel connected receivers. The receivers 1, 1 haveinput circuits similar to the one shown in connection with receiver 2,the input impedance of each being determined primarily by thepotentiometer.

In the operation of the arrangement shown in the figure the radioreceiver 2 is tuned to the desired frequency by suitable means includingthe variable condenser 20 and the volume of sound produced by the loudspeaker 23 is regulated by adjustment of the potentiometer 16. Theimpedance of the other similar receivers 1, 1 connected in common withthe transmission line 10 is effectively a shunt across the potentiometer16 of receiver 2 as shown diagrammatically by Z in the figure. Theseother receivers are subject to independent operation with respect bothto tuning and volume control. The impedance Z, therefore, is subject inthe general case to rapid and wide fluctuations in value. Some of thereceivers 1, 1 may be tuned to the same frequency as the receiver 2while others are tuned to different stations. In one case all thereceivers may be tuned to the same frequency. Each receiver whichresonates with receiver 2 takes a share of the energy of the desiredsignal intercepted by the antenna 13 thus tending to reduce the soundproduced by loud speaker 23 of receiver 2. On the other hand, eachreceiver which is tuned to a different station tends to act as a lowimpedance shunt across potentiometer 16. Thus, a receiver tuned toanother station may produce even greater efiect upon the sound in loudspeaker 23 than is produced by a receiver tuned to the same station.With a given setting or" the other receivers the potentiometer 16 ofreceiver 2 may be adjusted to give a suitable output of sound in loudspeaker 23. Upon readjustment of any other receiver, however, the volumeof sound in loud speaker 23 may undergo an undesired change requiringanother adjustment of potentiometer l6.

Disturbing as these changes in volume may be, it is found that they aremore readily controlled than are the effects upon the tuning ofcondensers 20 and 24. It is desired, of course, that condensers 20 and24 be aligned once and for all during the manufacture of the receivingsets so that thereafter the several tuned circuits may be simultaneouslytuned in a single mechanical operation. The setting of condenser 24 ispractically unafiected by changes in the impedance of other receivers onaccount of the shielding effect of the vacuum tube 21 whichsubstantially prevents interaction between its own input and outputcircuits. The setting of condenser 20 for a given station is, however,subject to variations according to the particular value of impedance Z.Therefore, if condensers 26 and 24 are adjusted properly forsimultaneous tuning with a given value of Z and this value is thereafterchanged, condenser 20 may become seriously detuned thereby decreasingthe sensitivity of the radio receiver and lowering the volume of soundproduced in the loud speaker 23.

It should be further noted that the transmission line ll) is terminatedin the impedance Z in parallel with the potentiometer 16. To maintainthe efiiciency and sensitivity of the high frequency transmission line,it is important that the terminal impedance should closely I1" tch thecharacteristic impedance of the transmission line. When this impedancematch is secured with a given setting of the radio receivers, the matchwill generally be disturbed when any receiver is readjusted and a lossof sensitivity will follow.

In order to obtain a substantial reduction in the detrimental effects ofvariations in the impedance Z, it is necessary to somewhat reduce thesensitivity of the initial stage in each radio receiver. In accordancewith the invention this is done in two ways, first by inserting theseries inductance 17 thus reducing the coupling between the transmissionline 10 and the transformer 18, and second, by making the impedance ofthe potentiometer 16 somewhat smaller than the impedance of circuit 11exclusive of the potentiometer. The use of an inductance instead of aresistance for reducing the coupling is found to .result in a smallerdecrease in sensitivity for a given reduction in the interaction betweenthe several receiving sets. The loosening of the coupling serves toreduce the effect of variations in the impedance of the first tunedcircuit When condenser 20 is adjusted. Further, the inductanceintroduces an increasing loss at the higher frequencies. This is adaptedto ofiset the rising sensitivity of the tuned circuit at thesefrequencies. The reduction of the impedance of potentiometer 16 servesto make the residual impedance variations of the tuned circuit ofrelatively small importance because they are shunted by the lowimpedance of the potentiometer.

What is claimed is:

l. A high frequency transmission system comrising an antenna, aplurality of radio receivng sets connected to said antenna and in par.llel with each other, each set including a tuned input circuit whichtends to make the set more sensitive at the higher radio frequencies andeach set including a series inductive coupling to coinpensate variationsof sensitivity caused by the tuned input circuit while at the same timesubstantially preventing interaction between the tuned input circuits ofthe several receiving sets.

2. A high frequency transmission system com prising a plurality of radioreceivers connected to a common antenna, each of saic. receiversincluding a coupling circuit characterized by an emciency whichincreases at the higher frequencies and each receiver including a seriesinductance element whereby the overall sensitivity of the individualreceiver is rendered substantially constant over a Wide range ofoperating frequencies and interaction between the several receivers issubstantially eliminated.

CHARLES B. AIKEN.

